1. Field of the Invention
The present invention is directed to a dynamic compressor for an analog signal to be compressed of the type employing a number of amplifier stages connected in succession.
2. Description of the Prior Art
The range of dynamics of magnetic resonance signals is very large but their relative bandwidth on the magnetic resonance frequency level is actually rather small. A dynamic range of approximately 100 dB is typical, as is an observation bandwidth of approximately 500 kHz. Despite the relatively small bandwidth, however, the signal processing is extremely complicated because of the high dynamics demand, and thus is cost-intensive. Accepting a moderate increase in the bandwidth, the costs can be lowered by means of a suitable compression and later expansion of the magnetic resonance signals and new approaches can be considered, particularly for wireless signal readout from local coils.
The expansion of the magnetic resonance signal can ensue after the digitization of the magnetic resonance signal. The expansion therefore can be implemented with relatively little cost expenditure, for example be reprogramming a memory that is already present anyway and in which correction values are stored. The compression, by contrast, must be implemented with the analog magnetic resonance signal. High demands are made of the input dynamics, definition and reproducibility of the characteristic, phase stability, etc. Further, the power consumption of the dynamics compressor also must be low given wireless signal readout.
UKW-Berichte 2/87, pages 66 through 87, discloses a dynamics compressor of the type. In this dynamics compressor, all amplifier stages exhibit the same stage limit level and the same stage amplification. This dynamics compressor exhibits a logarithmic characteristic.
xe2x80x9cDer Telefunken-Kompanderxe2x80x9d by Gerhard Dickopp and Ernst Schrxc3x6der, which appeared in Rundfunktechnische Mitteilungen, Volume 22 (1978), No. 2, pages 63 through 74, discloses a dynamics compressor for an analog signal to be compressed that has a number of series-connected amplifier stages. The analog signal is supplied to the first of the amplifier stages as an input signal and the output signal of that amplifier stage is supplied to the next amplifier stages as an input signal, and so on. Each amplifier stage outputs the input signal supplied thereto amplified with a stage amplification until the output signal reaches a stage limit level. This dynamics compressor has a logarithmic compression characteristic.
German OS 28 42 945 discloses a dynamics compressor that comprises an exponential compression characteristic.
The subsequent expansion becomes complicated given a logarithmic compression.
An object of the present invention is to provide a dynamics compressor with optimally low outlay, wherein the subsequent expansion is simpler to implement. This object is achieved in a dynamics compression of the type partially described wherein the magnitude of the curve of the sum signal above a minimal level roughly corresponds to an exponent characteristic of the magnitude of the analog signal.
The exponent of the exponent characteristic must, of course, be less than 1. It preferably lies between 0.2 and 0.5, particularly between 0.3 and 0.35. A value of one-third is quite specifically advantageous, i.e. when the exponent characteristic thus corresponds to a cube root.
Characteristics of this type are particularly advantageous because they proceed through the origin and are point-symmetrical. Both half-waves of a signal therefore are compressed to the same extent.
In practice, each exponent characteristic for arbitrarily small signals will have to be approximated by a straight line having a finite slope. Otherwise, an infinitely high amplification would have to be exerted in the proximity of the zero point. The minimal level therefore is defined by virtue of the transition from the linear region into the region of the exponent occurring at that level.
An exponent characteristic can be realized, for example, by the stage amplification of at least one of the amplifier stages differing from the stage amplifications of the other amplifier stages, and the stage limit levels of the amplifier stages being identical to one another.
Alternatively, it is possible for the stage limit level of at least one of the amplifier stages to differ from the stage limit levels of the other amplifier stages, for the stage amplifications of the amplifier stages to be identical to one another.
It is especially good for realizing the exponent characteristic for the stage limit levels of the amplifier stages to differ from one another in pairs. This is especially true when the stage limit levels monotonously decrease from the first to the last amplifier stage, particularly equidistantly.
It is generally true that the more amplifier stages employed the better the exponent characteristic can be approximated. Of course, the circuit-oriented outlay increases with the number of amplifier stages employed. A good compromise is for the dynamics compressor to have between four and ten, particularly between five and eight, amplifier stages, for example six amplifier stages.
The amplifier stages operate especially well when each of the amplifier stages has two steadily modulatable amplifier elements, each of which has one control terminal and two main terminals. Each of the amplifier elements has one of its main terminals connected via its own limiting resistor to a supply voltage and has the other main terminal connected via a constant current source to a reference potential. The input signal supplied to each amplifier stage is applied to the control terminals thereof and the signal output by each amplifier stage is taken between the main terminals and the limiting resistors.
When all of the limiting resistors exhibit the same value of resistance and the constant current sources of the amplifier stages output constant currents that differ from one another, then a relatively uniform structure of the individual amplifier stages is achieved.
Alternatively, it is possible that the limiting resistors of the amplifier stages exhibit values of resistance that differ from one another and all of the constant current sources of the amplifier stages output the same constant current.
When the input signal of the amplifier stages is symmetrically supplied thereto and when each of the amplifier stages symmetrically supplies its output signal to the summation stage, then even-numbered distortions are essentially suppressed.
When the summation stage is fashioned as a current node at its input side, then the summation of the output signals supplied by the individual amplifier stages is possible in an especially simple way. Dependent on the demands on the phase constancy between the analog signal and the sum signal, input resistors, low-pass filters or all-pass filters can be arranged between the summation stage and the amplifier circuits.
The inventive dynamics compressor is preferably employed for the compression of a magnetic resonance signal in a magnetic resonance coil.